Recently accompanying to the increased concern to the environment in global scale, a development of a fiber material decomposable in natural environment is strongly desired. For example, since the main starting material of conventional general purpose plastics is petroleum resource, it has become a big problem that the petroleum resource would be dried up in future, or that the global warming is caused by mass consumption of the petroleum resource.
For that reason, in recent years, research and development of various plastics and fibers such as aliphatic polyesters are activated. Among them, fibers made from plastics which can be decomposed by microorganism, i.e., biodegradable plastics have drawn attention.
In addition, by making plant resources as starting materials which take in carbon dioxide from the air to grow, not only it is expected to be able to control the global warming by circulation of carbon dioxide, but also it may be possible to solve the problem of the shortage of resources. For that reason, plastics of which starting materials are plant resources, i.e., plastics made by biomass have been drawing attention.
So far, biodegradable plastics made by biomass has problems that not only their mechanical properties and heat resistance are poor, but also their production cost is high, and they have not been used as general purpose plastics. On the other hand, in recent years, as a biodegradable plastic of which mechanical properties and heat resistance are relatively high and its production cost is low, polylactic acid of which starting material is lactic acid obtainable by fermentation of starch is in the spotlight.
Polylactic acid has been used for a long time, for example, in medical field as a sewing thread for surgical operation, but recently, by an improvement of its mass production technology, it became possible to compete in cost with other general purpose plastics. Accordingly, development of its commercial product as a fiber has been activated.
As developments of polylactic acid fiber, taking advantage of its biodegradability, agricultural materials, civil engineering materials or the like are going ahead. Following those, as big scale applications, apparel applications, interior applications such as curtain or carpet, automotive interior applications and industrial material applications are also expected. However, when it is used to the apparel applications or industrial material applications, the poor hydrolysis resistance of polylactic acid becomes a big problem. For example, when the polylactic acid fiber is made into a fabric and dyed in a high temperature dye liquid at 120 to 130° C. to dye deeply, its strength decreases to a fraction of its original due to a rapid progress of hydrolysis. Furthermore, when a polylactic acid fiber product is treated for a long time under a high temperature and high humidity which are resembled to those of a tropical area, similar to the case of dyeing process, the strength decreases by hydrolysis. The same appears in injection molded articles, i.e., hydrolysis advances as time goes, and accompanied with the decrease of molecular weight, the hydrolysis is accelerated to finally result in a fatal brittleness of the articles.
As method of improving weather resistance of the polylactic acid, method of preventing hydrolysis is general, for example, it is disclosed in JP 2000-136435 A, JP 2001-261797 A, JP 2001-323056 A, JP 2001-335626 A and JP 2003-301327 A. JP 2000-136435 A discloses a method of suppressing water content of polylactic acid to thereby prevent hydrolysis at fiber production process, JP 2001-261797 A, JP 2001-323056 A, JP 2001-335626 A and JP 2003-301327 A disclose fibers of which hydrolysis resistance is improved by, respectively, adding a monocarbodiimide compound, oxazoline compound, epoxy compound, polycarbodiimide compound. However, in the method of JP 2000-136435 A, although the hydrolysis in the production process could be prevented, any of characteristics of polylactic acid itself was not changed, i.e., its durability was poor. On the other hand, the carbodiimide compound of JP 2001-261797 A and JP 2003-301327 A, isocyanate originated from the starting material generates at melt-molding and there are problems that working environment at the production process grows worse, or the color tone of the obtained molded article becomes yellowish. Furthermore, as to the compound described in JP 2001-323056 A or JP 2001-335626 A, although the inventors traced, the capping effects to carboxyl end groups was insufficient and although their durabilities were improved to those to which the compound had not been added, it was still insufficient in practical use.